What we think about our material and social circumstances rather than just those circumstances alone, have a profound affect on our physical and mental health and on our life expectancy, and create social problems. It is not the average income of a society but the differences in income within a society that affects physical and mental health, once material needs are sufficiently met. Then relative income and relative deprivation have more powerful effects than absolute income. In most of the richer countries in which there is a large difference in status and income, there is a more hierarchical social structure and a more divided society.

Humans have evolved both to cooperate and to compete, to do things on behalf of others and for themselves. Societies of both human and non-human primates are found to be primarily organised either around affiliation, egalitarian reciprocity and co-operation or around power and dominance. People need to learn to survive in both types of society, organisation or group. Where the balance is skewed towards larger income differences the quality of life is affected for all.

Those lower down in the hierarchy experience more stress and anxiety. People compare themselves with others and can feel in a subordinate position or looked down on, and controlled or excluded from a better life. They can feel they have less autonomy and control over their lives, and so less able to pursue opportunities to achieve, feel competent, and improve their circumstances. The chances of upward mobility decrease. All this means that key social, emotional and self-esteem needs are not met: to be valued, noticed and heard (rather than ignored), to be respected and seen to be competent (rather than humiliated), to have enough freedom to get on with their lives, to be included, accepted and liked (rather than rejected). Individuals differ in how strong these needs are, and whether they express and act on these needs or not, but they are common to us all. (See for example the work of Bill Schultz ).

At a physical level when feeling excluded, inferior, controlled or restricted, unjustly treated, or rejected the pain centres of the brain are activated; when feeling respected and accepted by others, and connected and cooperating with them, the pleasure centres are activated. People low in status, with less control over their circumstances and more socially isolated are more prone to stress and physical and mental illness. In humans stress responses and emotional and mental processes become tuned and programmed to the environment.

Rates of mental illness were found by Wilkinson and Pickett to be five times higher in more unequal societies. Adversity suffered by parents on low income can create relationships at home that increase stress as it is, but infant mortality is lower in more equal societies. The benefits of smaller income differences spread across all income groups.

Social inequality heightens people’s sensitivity to, and anxieties about, social status as this becomes a more important indicator of self-worth and is seen as a key aspect of identity. Anxieties around how they are seen by others can make people inwardly more vulnerable and insecure. This can lead to “conspicuous consumption” (Veblen 1899) and wanting the same as others, and with credit cards, a debt-laden society suffering from hyper-consumerism. Or in extreme cases where people feel humiliated it can lead to rage and violent crime, as studies have shown. In more equal societies people have more opportunity to experience self-worth from meeting each other’s needs – feeling valued and included – alongside sufficient self-reliance, rather than from being higher in a status hierarchy.

The quality of social relations deteriorates as a society becomes more unequal and hierarchical. Fewer people can trust others or feel safe, there is more hostility, conflict and violence. There is less social cohesion; people choose friends from near equals. Empathy between status levels is less common where there is less similarity in material living standards (as de Toqueville found in the early 19th century). There is also more discrimination over gender, age, ethnic or social background, race, sexual orientation. This not only affects the quality of life but also creates more social problems (such as crime, drug abuse, teenage pregnancies, poorer educational attainment), costly to society and the taxpayer. When the quality of possessions becomes the marker of status and identity then striving to keep up with others in material goods also feeds into the excesses of consumerism.

Inequalities have grown in many of the rich countries over the last 25 years of last century and accompanying this there has been the longer transition from local community life to mass society with high geographical mobility over the last 50 years. People used to know and be known by people who remained part of a local community most of their life. Friends and a supportive social network that provides protection are all the more crucial where there are social inequalities and less sense of a local community.